Mobile communications products, such as cellular phone and handsets, are required to be small and light. Such products require radio frequency (RF) filters approximately covering the range 0.5 GHz to 10 GHz to protect the received signal from interference, either from the transmitter in the same handset and/or from unwanted externally generated signals. These filters must have low pass-band insertion loss (typically<2 dB) in order to achieve adequate signal-to-noise ratio. Due to their high quality factor, superior power handling capability, low cost packaging on silicon and potential for integration above IC, thin film bulk acoustic wave (BAW) resonators and filters have been widely used in mobile radio communication devices. The simplest implementation of a BAW resonator comprises a thin layer of piezoelectric material, for example, aluminum nitride (AlN), zinc oxide, and PZT, arranged between two metal electrodes. A BAW resonator typically is acoustically isolated from the supporting substrate by an acoustic isolator, which may include a cavity formed under a membrane supporting a BAW resonator or an acoustic mirror that includes of a stack of layers alternately formed of high and low acoustic impedance materials.
The resonant frequency of a BAW device is primarily determined by the thickness of all the layers included in the material stack upon which a resonator is fabricated. To date, available deposition equipments can hardly ensure a tolerance on layer thicknesses better than 1%. During BAW resonator fabrication, there can be a wide distribution of resultant resonant frequencies (e.g., it can be as large as 50 MHz) after initial wafer processing due to non-uniformity of film deposition, which lead to filters out of specifications and undesirably affect device yield. As a result, a wafer trimming process is typically utilized, wherein a determined amount of material is removed from the top layer (e.g., the passivation layer) of the multi-layer film stack to achieve a target BAW filter operating frequency across the wafer and from wafer to wafer, thereby, improve the manufacturing yield. In the case of AlN and SiN used as the trimming layer material, more than 100 nm thickness material may have to be removed to compensate for variations of the resonant frequency induced by process deviations.
FIG. 5 shows a conventional acoustic wave device 10 having a series acoustic wave resonator 14 and a shunt acoustic wave resonator 15 formed on a substrate 11, each having a piezoelectric layer 14b/15b sandwiched between a bottom electrode 14a/15a and a top electrode 14c/15c, and a passivation layer 14d/15d formed over the top electrode 14c/15c. Usually, the resonant frequencies of the shunt and series acoustic wave resonators differ by about 2% to about 7% and a mass load layer is added on the top electrode 15c of the shunt acoustic wave resonator 15 to shift its resonant frequency to a specified value relatively lower than the resonant frequency of the series resonator. However, as shown in FIGS. 6(a), 7(a) and 8(a), there is a considerable variation of the relative mass load effect versus the thickness of the trimming layer in the conventional acoustic wave device 10. This variation degrades the filter characteristics such as bandwidth and insertion loss, as shown in FIG. 9.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.